The hypothesis to be tested in this study is that prospective investigation of affected kindreds with MEN-IIa, MEN-IIb, or FMCT will allow early detection, syndrome delineation and improved survival. The polyendocrine neoplasms of MEN-II occur in two different inherited forms each with age-dependent biochemical and clinical penetrance: MEN type IIa (Sipple's syndrome) and MEN type IIb (multiple mucosal neurome syndrome). Medullary cancer of the thyroid (MCT), the most common index lesion of both MEN-IIa and MEN-IIb also occurs in a familial form, FMTC. This study, therefore, screens family members of probands for the potential expected endocrinopathy.